Enterprise compute and storage systems are increasingly deployed as modular systems with standardized form factor electronic enclosure modules mounted in standardized support structures. The standardized electronic enclosure modules may be devoted to perform any of a number of different functions such as computing, storage, or networking. The enclosure modules are commonly mounted in standardized support structures such as 19 inch (approximately 0.482 m) or 24 inch (approximately 0.610 m) wide racks. Such enclosures are commonly industry standard 1 U (1.75 inch; approximately 4.45 cm), 2 U (3.5 inch; approximately 8.89 cm), 3 U (5.25 inch; approximately 13.3 cm), or 4 U (7 inch; approximately 17.8 cm) high. Often, the reasons for the adoption of the larger 2 U, 3 U, or 4 U modules is to increase reliability through improved airflow for cooling and to provide space for more adapter cards.
Such modular enclosures are customarily air-cooled. They draw air in from the room they are housed in by means of fans that accelerate the air and force it over the enclosure's internal components to cool them. The resulting heated air is exhausted back into the room. The room air itself is circulated through an air cooler or a Computer Room Air Conditioner (CRAC) that is, in turn, cooled by a refrigeration system. Even for moderately powered systems, very large volumes of air must be moved from the room through the modules, racks, and CRACs. Fans commonly account for 25% of the total power consumed in the modules and racks. CRAC fans consume another 0.1 watt per watt of load. This cooling burden is passed to the refrigeration system that consumes another 0.3 to 0.4 watts per watt of load. The latter load might be increased by hot and cold air mixing in the room, further reducing cooling efficiency. All these effects, together with electrical power conversion and distribution losses, require that, for every watt of power consumed by the computing section of a server, typically 2.8 watts must be supplied to a modern best-in-class data center. In many data centers, up to 4 watts must be supplied.
In spite of the large amount of energy expended on moving the air, the thermal resistance from the electronic devices internal to a modular electronic enclosure to the cooling fluid passing through the air coolers is still excessively high, typically 0.5° C./watt to 0.7° C./watt. This results in a large temperature drop from the devices to the cooling fluid. For example, a 120 watt processor with a path having a thermal resistance of 0.5° C./watt to the cooling fluid produces a thermal drop of 60° C. In order to maintain a device case temperature of 70° C., the cooling fluid temperature cannot be higher than 10° C. This requires a refrigeration cycle that absorbs considerable energy.
If the thermal resistance could be lowered then the temperature of the cooling fluid could be increased resulting in an improvement of the thermal efficiency of the entire cooling infrastructure. In some cases, the permissible temperature of the cooling fluid could be increased sufficiently for the refrigeration system to be replaced by a natural cooling system such as that provided by the evaporation of water in a cooling tower or dissipation to groundwater.
Although fluids are sometimes used in cooling electronics, no fully integrated, modular, reliable, simple, and cost effective solution has emerged. Issues to overcome include: difficult installation and maintenance; modularity and scalability; decreased reliability due to numerous fluid connections; difficulty in applying the technology to existing products and environments; and establishing a low thermal impedance path from the device-to-be-cooled to an external chiller.